In the preparation of concrete bodies, particular slabs for road or bridge repair, the surface of the fresh concrete is first cleared of the aqueous solution/suspension (hereinafter referred to as "water") arising from the concrete. Then further water is drawn from the capillaries of the porous body to maximize its hardness. As much of this water as possible is drawn off, but under any circumstances more is likely to diffuse into surface layers from more deeply within the body. Whether the concrete in question is of standard mix or of the new synthetic-resin colloidal types, it is normally desired to reduce the moisture content to less than 5% of the mass of the body. This is fairly difficult to do, especially outside where humid surroundings can impede drying of the body, or where in fact the body might be rained on or subject to other wet climatic action. Capillary action from dew or rainfall alone can draw considerable moisture relatively deep into a fresh concrete body.
When excessive moisture is left in the concrete body, bubbles can form, even in relatively moisture-tolerant resin mixes. In situations where the body must be bonded to an existing body, as in a repair job, the resultant bond can be weak, leading to early separation of the new from the old.
The standard method of drying a fresh concrete body, that is one that was recently placed or poured and that has not fully cured, is by the simple application of heat to its upper surface. Heating the surface of the body produces a vapor-pressure gradient from the outside of the body inward, with the dew point being further inward when the surface is hot. Thus as the body cools, the dew point moves outward so that in effect moisture is caused to diffuse from the interior toward the surface of the body. Added to this effect is the moisture supplied by the surroundings in the form of humid air, precipitation, dew, or the like, and the fresh concrete can quickly take on an unacceptable amount of water. Putting a vapor barrier on the surface, typically a plastic sheet, merely drives back water rising to the surface of the concrete.